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Thread: "Grabby" or "jerky" brakes?

  1. #1
    Seattle-area Rounder OfficerImpersonator's Avatar
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    "Grabby" or "jerky" brakes?

    I bought my 2002 RTP from a private party five years ago. Never having ridden an R-bike with ABS brakes before, I assumed the "grabby/jerky" brakes were simply a quirk of the machine.

    Over the next couple of months of riding, I learned that the "grabbiness/jerkiness" of the brakes could be eliminated by proper modulating of the front and rear brakes - and I've been employing that strategy for the last five years.

    Last week, I was having my front tire changed. While the wheel was off the bike, I was carefully inspecting the rim and brake rotors. I noticed that my ABS sensor ring had a tiny dent, where two of the metal cut-outs in the sensor ring were pushed out perhaps by three millimeters. I pondered this situation for a moment, and theorized that perhaps the ABS sensor counts the spaces in the ring as it goes around, and these two spaces aren't "seen" by the sensor, so the ABS system thinks the wheel has locked up for a microsecond - and thus releases the brake pressure, causing the jerky stops.

    I used a rubber mallet to gently tap the back of a screwdriver placed against the bent cogs on the sensor ring and slowly and gently pushed the cogs back into place. In a few moments, I had the spaces on the ring back in perfect order.

    I was completely unprepared for what a completely different bike I now have. The brakes are now flawless - strong, smooth, linear and powerful. No more jerking to a stop. No more sudden foot stabs when the bike lurches to a stop a few feet prior to the anticipated stopping point. I had to take it out this past Sunday just to see how much better things are. I can now brake much cleaner through corners, and I'm much more confident maneuvering >5 mph.

    I ride 12K miles a year, so it's not like I'm a rookie or anything. I simply never imagined that my problem - to which I had adapted my riding style - could so easily be rectified!

    In the event that you find your ABS brakes behaving strangely, and if you in particular think your brakes are "grabby" or "jerky", I suggest a careful inspection of your ABS sensor rings. If the damage is minimal, the repair can be easy and the rewards amazing.
    Seattle, WA
    2012 R1200GSA
    2002 R1150RT-P
    1992 K75S sold

  2. #2
    BUBBAZANETTI
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    veeeery interesting. I'm surprised how much more "grabby" my servo assisted 04's brakes are than my 99's were.......not necessarily in a good way.

  3. #3
    Outlander Omega Man's Avatar
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    Quote Originally Posted by OfficerImpersonator View Post
    I noticed that my ABS sensor ring had a tiny dent, where two of the metal cut-outs in the sensor ring were pushed out perhaps by three millimeters. I pondered this situation for a moment, and theorized that perhaps the ABS sensor counts the spaces in the ring as it goes around, and these two spaces aren't "seen" by the sensor,
    Nice catch
    "Well they say.. time loves a hero but only time will tell.. If he's real, he's a legend from heaven If he ain't he was sent here from hell" Lowell George
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  4. #4
    Registered User awagnon's Avatar
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    It sounds like your brakes were excessively "grabby" and improved by straightening out the sensor ring. I don't know about the 2002 RTP, but the early 2002 RT had notoriously grabby brakes which BMW fixed later that year. Mine are the grabby ones and nearly threw me over the handlebars the first time I rode it.
    Al - Ogden, Utah
    Boxer bikes, boxer dogs, and letterboxing.
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  5. #5
    Seattle-area Rounder OfficerImpersonator's Avatar
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    Quote Originally Posted by awagnon View Post
    It sounds like your brakes were excessively "grabby" and improved by straightening out the sensor ring. I don't know about the 2002 RTP, but the early 2002 RT had notoriously grabby brakes which BMW fixed later that year. Mine are the grabby ones and nearly threw me over the handlebars the first time I rode it.
    I presumed that was the case with my 2002, and that's why I accepted the brakes as they were for four and a half years. I just assumed that's how they were supposed to be because lots of other people were venting about grabby/jerky/"too-powerful" brakes.

    If I were you, I'd spend 30 seconds looking over the ABS sensor ring just to make sure it's pristine. You don't have to remove any parts to see it - just put the bike on the center stand and rotate the front wheel, all the while inspecting the sensor ring for defects.
    Seattle, WA
    2012 R1200GSA
    2002 R1150RT-P
    1992 K75S sold

  6. #6
    Nickname: Droid
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    Good catch on the deformed sensor ring. First, your assumption about the sensor "counting" the spaces is correct. As the metal leading and trailing edge of each solid piece of the ring passes the magnetic field coil in the sensor, it disrupts the magnetic field and generates a rising/falling signal output from the sensor. It actually creates an electrical AC sine wave output. The wheel speed sensor becomes a tiny AC generator of a sort.

    The ABS computer "clips" the signal into a uniform value square wave form, which creates a rising/falling square wave form signal. This signal very accurately relates to the actual wheel speed. Both the front and rear sensor/wheel system creates a signal that the ABS computer compares to determine if one wheel, WHEN BRAKING IS APPLIED, may be turning slower than the other wheel. Beyond a preset speed thresehold differential, again only with brake pressure applied, the ABS system engages wheel speed/braking control over the input from the rider.

    Now, under normal braking the ABS system does nothing but monitor wheel speeds. IF the speed differential is exceeded during braking the ABS engages control. So normally all normal braking conditions cause no ABS control. But if a deformed wheel sensor ring could create the signal differential, on bikes with Servo-assist ABS systems, then the ABS may attempt control under braking conditions where wheel slide is not likely or happening at all. The deformed sensor wheel can cause a false ABS control condition and the ABS system may engage control to adjust the wheel speeds to match. But the deformed signal causes the ABS controller to think the wheel speed has not matched the other wheel, and continues control until the minimum low speed thresehold is achieved the system disengages.

    I'm not sure that a similar deformed speed sensor condition under normal (non ABS) braking would create the braking reaction you describe on the older generation ABS system like that on my 94 R1100RS. But the servo assisted ABS system is very different and could engage ABS control during normal braking if a false speed sensor signal is caused by a deformed wheel ring. Especially so if the deformed portion of the wheel ring is a small section of enough "teeth" to cause a momentary ABS control sequence, but then no ABS control for the rest of the wheel rotation. An on-off ABS control condition, which could cause very jerky braking.

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